The present invention relates to an optical scanning device comprising an optical objective lens, and to an optical element comprising an objective lens. More specifically, but not exclusively, the invention relates to an optical scanning device capable of writing and/or reading data to and/or from an optical record carrier such as a digital versatile disc (DVD), with a relatively large field tolerance using a single optical objective lens system. The field tolerance of a lens is limited by comatic aberrations introduced at off-axis parts of the field.
Objective lenses with relatively large field tolerances are desirable for use in scanning devices which allow multi-track readout, and/or radial tracking, in which the position of a beam spot varies in relation to the objective lens. For multi-track readout of DVDs, objective lenses with a numerical aperture (NA) at least of NA=0.6 and large field tolerances are required. Using standard glass replication techniques, only one surface of the lens may be made aspherical, significantly reducing the potential field tolerance of the lens. With known glass moulding or injection moulding techniques, two surfaces of a lens can be made aspheric. Although this in principle makes it possible to design a lens with a large field tolerance, allowing sufficient tolerance for tilt and decentring between the two aspheric surfaces of the lens again leads to a limited field tolerance. Consequently, manufacturing a lens with a large field, meeting the various constraints inherent in modern optical scanning devices, is difficult.
Using periodic (i.e. regularly repeating) phase structures, which provide diffractive effects, on the surface of a lens, it is possible to introduce comatic aberrations, whereby comatic aberrations generated in the lens may be compensated for. However, such diffractive structures are difficult to manufacture and are wasteful of radiation, causing a significant amount of scattering.
In accordance with one aspect of the present invention there is provided an optical scanning device for scanning optical record carriers with radiation of a selected wavelength, the device including an objective lens, having an axial direction and a radial direction, and a phase structure which is non-periodic with respect to the radial direction, the non-periodic phase structure being arranged to compensate for comatic aberrations generated in the objective lens when an optical record carrier is read in a direction which is non-axial with respect to said objective lens, whereby an improved field of view is provided for said objective lens.
The root mean square (rms) comatic wavefront error caused by the objective lens at a certain field angle with respect to the axial direction is preferably compensated for by at least 50% by said non-periodic phase structure. More preferably, the compensation is at least 70%. The field angle for an optical recording read and/or write device is preferably 1xc2x0. This is also the maximum required field angle for multi-track scanning of an optical record carrier. The resulting apparatus provides a significantly improved performance with a relatively large field tolerance. In the field of optical recording the residual amount of wavefront error at the maximum required field angle is after compensation preferably less than 40 mxcex and more preferably less than 20 mxcex.
The non-periodic phase structure may include a plurality of annular zones, each of said zones comprising a step of a substantially constant height with respect to a rotationally symmetrical aspheric shape generally followed by said objective lens. Steps in the non-periodic phase structure preferably generate a relative phase difference of approximately a multiple of 2xcfx80 for radiation of said selected wavelength when an optical record carrier is read in said axial direction. Thereby, the effect on the performance of the lens when operating axially is not significant.
The heights of said zones are selected substantially optimally in relation to the comatic aberration to be compensated for. Thereby, a relatively large degree of compensation can be provided.
The number of zones in the non-periodic structure is preferably limited, for manufacturing efficiency. The number of said zones is preferably less than 10, and may be in the region of only 5.
In accordance with a further aspect of the present invention there is provided an optical including an optical element having optical power and an axial direction and a radial direction, and a phase structure which is non-periodic with respect to the radial direction, the non-periodic phase structure being arranged to compensate for comatic aberrations generated by the optical element when an optical beam traverses the optical system in a direction which is non-axial with respect to said element, whereby an improved field of view is provided for said optical element. The optical element may be a lens or a mirror.
It is noted that it has previously been proposed to use a non-periodic phase structure to compensate for wavefront aberrations. Reference is made to xe2x80x9cDual-wavelength optical head with a wavelength-selective filter for 0.6- and 1.2-mm-thick-substrate optical disksxe2x80x9d, Katayama et al., Applied Optics, Vol. 38, No. 17, Jun. 10, 1999, JP-A-11002759 and EP-A-865037. However, in these proposed arrangements the non-periodic phase structures are used in order to compensate only for spherical aberrations due to differences in information layer depths in optical disks, when using two different wavelengths of radiation. Coma compensation is not considered or provided for.
Note that, in the case of the present invention, comatic aberration compensation may be achieved without altering the wavelength of the radiation applied.